Key points
Increasing the total organic carbon in soil may decrease atmospheric carbon
dioxide and increases soil quality.
The amount of organic carbon stored in soil is the sum of inputs to soil (plant and
animal residues) and losses from soil (decomposition, erosion and offtake in plant
and animal production).
Biology
HOW MUCH CARBON CAN SOIL
STORE?
The maximum capacity of soil to store organic carbon is determined by soil type
(% clay).
Management practices that maximise plant growth and minimise losses of organic carbon
from soil will result in greatest organic carbon storage in soil.
Recent interest in carbon sequestration has raised
questions about how much organic carbon (OC) can be
stored in soil. Total OC is the amount of carbon in the
materials related to living organisms or derived from them.
In Australian soils, total OC is usually less than 8% of total
soil weight (Spain et al., 1983) and under rainfed farming
it is typically 0.7–4%. Increasing the amount of OC stored
in soil may be one option for decreasing the atmospheric
concentration of carbon dioxide, a greenhouse gas.
Increasing the amount of OC stored in soil may also improve
soil quality as OC contributes to many beneficial physical,
chemical and biological processes in the soil ecosystem
(figure 1) (see Total Organic Carbon fact sheet). When OC in
soil is below 1%, soil health may be constrained and yield
potential (based on rainfall) may not be achieved (Kay and
Angers, 1999).
Chemical
• Buffers pH
• Complexes cations
• Immobilises pollutants
• Binds heavy metals
Biological
TOTAL
OC
• Energy for
biological processes
• Large store of
nutrients—N, P, S, K, Ca,
Cu, Zn etc.
• Improves soil
resilience
Physical
• Improves soil structural
stability
• Influences water retention
• Mulching reduces water/soil loss
• Buffers soil temperature
Figure 1: Some of the beneficial physical, chemical and biological
processes in soil that total OC contributes to.
Carbon budgets in soil—Inputs and
losses of organic carbon
The amount of OC stored in soil is the difference between
all OC inputs and losses from a soil. The main inputs of OC
to soil in rainfed farming systems are from plant material,
such as crop residues, plant roots, root exudates and animal
manure. Inputs of plant material are generally higher when
plant growth is greater.
Losses of OC from soil are from decomposition by
microorganisms, erosion of surface soil and offtake in plant
and animal production. Decomposition occurs when
microorganisms use OC in soil to obtain the carbon,
nutrients and energy they need to live. During
decomposition, OC is lost from soil because
microorganisms convert about half of the OC to carbon
dioxide gas (CO2). Without continual inputs of OC, the
amount stored in soil will decrease over time because OC is
always being decomposed by microorganisms.
Losses of OC from erosion of surface soil can have a large
impact on the amount of OC stored in soil. This is because
OC is concentrated in the surface soil layer as small particles
that are easily eroded. In Australian agriculture, erosion
can cause the annual loss of 0.2 t/ha of soil from a pasture,
8 t/ha from a crop and up to 80 t/ha from bare fallow.
Offtake of OC in plant and animal production is also an
important loss of OC from soil. Harvested materials such as
grain, hay, feed and animal grazing all represent loss of OC
(and nutrients) from soil.
Soil type determines the potential
storage of organic carbon
The potential storage of OC in soil depends on the soil
type (figure 2). Clay particles and aggregates can reduce
losses of OC by physically protecting organic matter from
How much carbon can soil store?
Background
Potential OC
Attainable OC
Actual OC
Soil type
Climate
Clay content, bulk
density, depth,
mineralogy
Rainfall,
temperature,
solar radiation
Plant productivity,
Management rotation strategy,
soil management
Organic carbon storage in soil
Figure 2: The influence of soil type, climate and management
factors on the storage of organic carbon (OC) that can be
achieved in a given soil. Based on Ingram and Fernandes (2001).
decomposition. Particles of organic matter can become
adsorbed to clay surfaces, coated with clay particles
or buried inside small pores or aggregates. All of these
processes make it difficult for microorganisms to come in
contact with organic matter. Therefore, the amount of OC
stored in soil tends to increase with increasing clay content
(figure 3). In contrast, in sand soil microorganisms are able
to more easily access OC. This causes greater loss of OC by
decomposition.
The potential storage of OC in soil is rarely achieved
because climate reduces inputs of OC to soil.
Figure 3: The relationship between clay content and the organic
carbon content of 220 soils in a 10 hectare area of a paddock
under cereal-legume rotation in the central agricultural region
of Western Australia.
Management determines the actual
storage of organic carbon in soil
Management practices determine the actual storage of OC
in soil by increasing inputs and decreasing losses (figure 2).
Practices that can increase the amount of total OC stored
in soil include:
Climate determines the attainable
storage of organic carbon
Climate determines the attainable storage of OC in soil
by regulating plant production (figure 2). Under dryland
agriculture, rainfall is the climate factor that has most
influence on plant productivity and therefore inputs of
OC to soil. In regions with high rainfall, soils tend to have
greater attainable storage of OC than the same soil type in
a lower rainfall region.
Although it is not possible to increase the attainable storage
of OC in soil, management practices determine whether or
not the attainable storage of OC in soil is achieved.
•• Increased plant growth generally increases inputs of
OC to soil in shoot material, roots and root exudates,
e.g. optimal nutrition, increasing water use efficiency,
decreasing disease.
•• Growing plants for longer periods each year generally
increases inputs of OC to soil, e.g. shorter fallow,
conversion from cropping to pasture, conversion from
annual to perennial pasture.
•• Improving soil structure can increase the amount of
OC stored in soil by reducing losses of OC from soil
by decomposition and erosion, e.g. retaining stubble,
maintaining ground cover and reducing compaction by
vehicles and stock.
How much carbon can soil store?
References
This factsheet was based on: Hoyle F, Murphy D and Baldock G (2009) Soil organic carbon: Role in Australian farming
systems. Book chapter, in press.
Ingram JSI and Fernandes ECM (2001) Managing carbon sequestration in soils: Concepts and terminology. Agriculture,
Ecosystems & Environment 87: 111-117.
Kay BD and Angers DA (1999) Soil structure. In: ‘Handbook of Soil Science’ (Ed ME Sumner), pp A-229-276. CRC Press, Boca
Raton USA.
Spain AV, Isbell RF and Probert ME (1983) Soil organic matter. In: ‘Soils—An Australian viewpoint’, pp 551-563. CSIRO,
Melbourne Australia. Academic Press, London UK.
Author: Jennifer Carson (The University of Western Australia)
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Wheatbelt NRM under the Caring for our Country Program.
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